1. Field of the Invention
The present invention relates to an automotive anti-skid brake control system generally abbreviated as an "ABS", and specifically to a brake-fluid reflux type anti-skid brake control system which can provide a wheel-cylinder pressure moderately intensifying characteristic at an ABS pressure intensifying mode at which the wheel-cylinder pressure is intensified during operation of the ABS. Such a wheel-cylinder pressure moderately intensifying characteristic will be hereinafter referred to as an "ABS moderate pressure intensifying characteristic".
2. Description of the Prior Art
Recently, there have been proposed and developed various anti-skid brake control systems for preventing brakes from locking road wheels during quick braking or during braking on a low frictional road surface so as to prevent skidding. In large luxury cars and high grade vehicles, the ABS is often assembled to provide maximum effective braking, and consequently to decrease a braking distance and to enhance a directional stability of the vehicle, irrespective of the road condition. The ABS permits normal application of the brakes by alternately holding, reducing, holding, and intensifying the wheel-cylinder pressure, such that braking can be held to just below the point at which a skid would start to develop.
As is generally known, the ABS comes into operation if an electronic control unit detects that the vehicle wheel is locked during quick braking or during braking on a low frictional road surface. At the beginning of the anti-skid brake control, the ABS operates at a wheel-cylinder pressure reducing mode which will be referred to as an "ABS pressure reducing mode". In the ABS pressure reducing mode, the brake fluid in the wheel cylinder is temporarily stored in a brake fluid reservoir through a directional control valve arranged in a brake fluid reflux line. Thereafter, a fluid-pressure pump is driven in order to reflux the brake fluid in the reservoir to a hydraulic pressure supply line connected to the outlet port of the master cylinder, with the result that the wheel-cylinder pressure is gradually reduced. Since the above fluid pressure pump is driven only during operation of the ABS, the fluid pump is generally referred to as an "ABS pump". As set forth above, the wheel-cylinder pressure is reduced in accordance with the ABS pressure reducing mode. Thereafter, when the control unit detects the road wheel has been unlocked during operation of the ABS, a brake fluid introduced from the master cylinder side and a brake fluid discharged from the ABS pump and refluxed just downstream of the outlet port of the master cylinder are both fed through a directional control valve serving as a pressure intensifying valve to the wheel cylinder. Thus, the wheel-cylinder pressure is intensified again. In this manner, the wheel-cylinder pressure is suitably adjusted by the ABS, so as to effectively prevent an undesirable wheel-lock during quick braking or during braking on a low frictional road surface, while providing a reasonably high braking force.
In the prior art ABS not including an ABS moderate pressure intensifying characteristic, after the wheel-cylinder pressure control mode is changed from the ABS pressure reducing mode to the ABS pressure intensifying mode, there is a tendency for the brake fluids output from the master cylinder and the ABS pump to be rapidly fed into the wheel cylinder. Thus, there is a possibility that the wheel is locked again, due to an excessive pressure-rise. In order to solve such a problem, a more recent anti-skid brake control system employs a brake-fluid flow control valve which is arranged in the brake-fluid supply line between the master cylinder and the wheel cylinder for assuring ABS a moderate pressure intensifying characteristics at an ABS pressure intensifying mode. Such ABS with a moderate pressure intensifying characteristics has been disclosed in Japanese Patent Second Publication No. 61-16657 (corresponding to U.S. Pat. No. 4,453,782) and Japanese Patent First Publication No. 56-82650 (corresponding to German Patent Application No. P 29 45 444.2). Such a more recent ABS employing the above brake fluid flow control valve with the ABS moderate pressure intensifying characteristic is shown in FIG. 8.
Referring now to FIG. 8, the conventional anti-skid brake control system includes a hydraulic brake circuit (corresponding to the right-half of FIG. 8) of a front wheel side for independently controlling front-right and front-left brakes and a hydraulic brake circuit (corresponding to the left-half of FIG. 8) of a rear wheel side for commonly controlling rear-right and rear-left brakes. Since the basic construction of the front-wheel-side hydraulic brake circuit is similar to that of the rear-wheel-side hydraulic brake circuit, only the rear-wheel-side brake circuit will be hereinafter described in detail for the purpose of simplification of description. In FIG. 8, the same reference numerals used in the front-wheel-side brake circuit will be applied to the corresponding elements used in the rear-wheel-side brake circuit. The conventional anti-skid brake control system includes a main brake-fluid supply line A arranged for supplying brake fluid from a master cylinder 1 to a wheel cylinder 2 during usual braking, and a brake-fluid reflux line B arranged in parallel to the supply line A for returning the brake fluid from the wheel cylinder 2 just downstream of the outlet port of the master cylinder 1. The conventional ABS includes a first electromagnetic directional control valve 3 arranged in the supply line A and mainly serving as a pressure intensifying valve, and a second electromagnetic directional control valve 4 arranged in the reflux line B and mainly serving as a pressure reducing valve. The ABS also includes a brake-fluid reservoir 5 and an ABS pump 6 in the reflux line B. With the above arrangement, the brake fluid in the wheel cylinder 2 is temporarily stored in the reservoir 5 by means of the ABS pump 6 having a driven connection with an electric motor M at the ABS pressure reducing mode and thereafter the brake fluid stored in the reservoir 5 returns just downstream of the outlet port of the master cylinder through rotation of the ABS pump at the ABS pressure intensifying mode. As clearly shown in FIG. 8, the first directional control valve 3 has a fixed orifice 7 so as to assure the ABS moderate pressure intensifying characteristics in the ABS pressure intensifying mode. The ABS operates to provide a maximum effective braking by alternately holding, reducing, holding, and intensifying the wheel-cylinder pressure if an electronic control unit (not shown) detects that the road wheel is locked during quick braking or during braking on a low frictional road surface. As seen in FIG. 8, since the normally open directional control valve 3 is opened and the normally closed directional control valve 4 is closed during usual braking, the orifice 7 provides a suitably restricted fluid flow through an orifice constriction even during usual braking. As a result, a braking response is deteriorated during usual braking due to the above orifice constriction. It is difficult to design a predetermined restricted fluid flow rate of a fixed orifice assuring a desired ABS moderate pressure intensifying characteristics, without deteriorating the braking responsiveness during usual braking.